Urban population by city size is determined by population density and commuting patterns; this better reflects the economic function of cities in addition to their administrative boundaries. Urban areas in OECD countries are classified as: large metropolitan areas if they have a population of 1.5 million or more; metropolitan areas if their population is between 500 000 and 1.5 million; medium-size urban areas if their population is between 200 000 and 500 000; and, small urban areas if their population is between 50 000 and 200 000. This indicator is measured as a percentage of the national population.